@Leadfoot,
The article wasn't ONLY about moving electric fields. Just look at the form of the question:
" The original question was: An electric field stores energy. Energy has mass if I understand E=mc correctly..."
And look at the form of the answer:
"Physicist: You’re exactly right. The electric field has mass (or, at the very least you could say that it has inertia and attracts things gravitationally), because it carries energy. The energy density, K, of the electric field around a charge, q, is..."
That's plain enough: an electric field stores energy and has mass/inertia.
Also see this Feynman lecture on renormalization theory, which also mentions moving electrons but (not surprisingly) begins with unmoving electrons (and pay close attention to the sentence I've taken the liberty of placing in all-caps):
"First, we compute the energy of a charged particle. Suppose we take a simple model of an electron in which all of its charge [Math Processing Error] is uniformly distributed on the surface of a sphere of radius [Math Processing Error], which we may take to be zero for the special case of a point charge. Now let’s calculate the energy in the electromagnetic field. If the charge is standing still, there is no magnetic field, and the energy per unit volume is proportional to the square of the electric field. The magnitude of the electric field is [Math Processing Error], and the energy density is [Math Processing Error]. TO GET THE TOTAL ENERGY, WE MUST INTEGRATE THIS DENSITY OVER ALL SPACE."
http://feynmanlectures.caltech.edu/II_28.html
Note that "math processing error" seems to have been added by my cellphone: I can't imagine it's present in the original text!
P.S. Potential energy is negative (mathematically speaking), so I think the electric field decreases the mass of the electron as a whole.